The seasonal timing of agricultural ammonia application is primarily controlled at the present time by economic factors. Due to the short growing period in much of the corn growing region, application in the fall season is most common, and then typically an additional application in the spring season prior to planting is attempted. Occasionally, however, weather conditions prevent a spring pre-plant application, such that an application into the sides of the rows after planting, known as a "side dress application", is required.
Another factor which may effect the seasonal timing of ammonia applications relates to increasing awareness that heavy fall season applications of ammonia may have an effect on streams and reservoirs of surface water runoff from fields where ammonia has been applied. The runoff water from ammonia-enriched fields can in some situations upset the nutrient balance in the streams and reservoirs causing distortions in plant and animal life. Thus, it is believed likely that quantity limits per application will be imposed by government regulation in some geographical growing areas, which in turn will encourage increased utilization of side dressing applications.
A problem that has arisen with side dress applications, however, arises from the fact that the injection of ammonia in side dressing is necessarily shallower than in conventional fall and spring pre-plant applications. Shallow injection would be acceptable if the ammonia stream were primarily in the form of a liquid with little vapor, but the typical high vapor to liquid ratio results in the escape of ammonia, causing unacceptable leaf burn. Ammonia, being a liquified vapor, has a great expansion rate from the tank to the applicator knives where it is injected in the soil, and typically 70-75% of the ammonia by weight represents less than 1% of the total volume injected.